The increasing demand for quantity and speed of data constantly puts higher requirements on all available media for accommodating the increasing demand. Noticeably, data transmission at short distances is now a topic of great interest. In order to ensure the operation according to current technical standards and regulations, monitoring and diagnostics of the deployed network are necessary. Of course, the techniques applied should be reliable and relatively inexpensive.
Existing communication networks comprise e.g. copper cables, both newer and older copper cables. Depending on the quality of the cables, they are more or less capable of supporting both high speed data transmission and a large amount of concurrent data transmission.
One way of monitoring a link is determining a loop by means of which all loop components may be discovered, but determining their length and respective electrical parameters are still the challenging task. Certain methods are complementary or partly complementary to each other, like Single Ended Loop Test, SELT, and Double Ended Loop Test, DELT. SELT and DELT may be performed in both time and frequency domain. Mathematically they are equivalent since time and frequency domains are related through Fourier transform. Just for sake of explanation consider Time Domain Reflectometer (TDR). A short pulse is transmitted through the cable. Any impedance discontinuities will cause some of the incident signal to be sent back towards the source. This is similar in principle to the radar. The resulting reflected signal that is measured at the output/input to the TDR is displayed or plotted as a function of time and, because the speed of signal propagation is almost constant for a given transmission medium, can be read as a function of cable length. There are many quite complicated problems related to retrieving the information on network topology partly due to noise, partly to low accuracy, partly to imperfect model of cable.